chore: automate releasing to Maven central when a tag happens. (#197)

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Fixes #<issue_number_goes_here> 🦕

Author: kabir

.github/workflows/release-to-maven-central.yml

@@ -0,0 +1,52 @@
+name: Publish release to Maven Central
+
+on:
+  push:
+    tags:
+      - 'v?[0-9]+.[0-9]+.[0-9]+*' # Trigger on tags like v1.0.0, 1.2.3, v1.2.3.Alpha1 etc.
+
+jobs:
+  publish:
+    # Only run this job for the main repository, not for forks
+    if: github.repository == 'a2aproject/a2a-java'
+    runs-on: ubuntu-latest
+    permissions:
+      contents: read
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up JDK 17
+        uses: actions/setup-java@v4
+        with:
+          java-version: '17'
+          distribution: 'temurin'
+          cache: maven
+
+      # Use secrets to import GPG key
+      - name: Import GPG key
+        uses: crazy-max/ghaction-import-gpg@v6
+        with:
+          gpg_private_key: ${{ secrets.GPG_SIGNING_KEY }}
+          passphrase: ${{ secrets.GPG_SIGNING_PASSPHRASE }}
+
+      # Create settings.xml for Maven since it needs the 'central-a2asdk-temp' server.
+      # Populate wqith username and password from secrets
+      - name: Create settings.xml
+        run: |
+          mkdir -p ~/.m2
+          echo "<settings><servers><server><id>central-a2asdk-temp</id><username>${{ secrets.CENTRAL_TOKEN_USERNAME }}</username><password>${{ secrets.CENTRAL_TOKEN_PASSWORD }}</password></server></servers></settings>" > ~/.m2/settings.xml
+
+      # Deploy to Maven Central
+      # -s uses the settings file we created.
+      - name: Publish to Maven Central
+        run: >
+          mvn -B deploy
+          -s ~/.m2/settings.xml
+          -P release
+          -DskipTests
+          -Drelease.auto.publish=true
+        env:
+          # GPG passphrase is set as an environment variable for the gpg plugin to use
+          GPG_PASSPHRASE: ${{ secrets.GPG_SIGNING_PASSPHRASE }}

pom.xml

@@ -62,6 +62,9 @@
 
         <!-- Redirect test output to file -->
         <maven.test.redirectTestOutputToFile>true</maven.test.redirectTestOutputToFile>
+
+        <!-- release properties -->
+        <release.auto.publish>false</release.auto.publish>
     </properties>
 
     <dependencyManagement>
@@ -206,6 +209,7 @@
                     <configuration>
                         <!-- This should point to a server entry in your settings.xml -->
                         <publishingServerId>central-a2asdk-temp</publishingServerId>
+                        <autoPublish>${release.auto.publish}</autoPublish>
                     </configuration>
                 </plugin>
                 <plugin>